The magic round in England is held in places like Newcastle, Manchester and Cardiff - places with big stadiums and tourist boards who will pay big money to get away fans from 12 clubs plus all the neutrals to come to their town for a big weekend.
The obvious place to hold the magic round would be Melbourne, but Perth, Adelaide and even somewhere like Wellington in NZ would be a good option.

Article about Newcastle bidding to hold the magic round.
Says Qld has it for the next 2 years with an option for another 2 years and the Qld gov pays 2.5 mill a year.

I think Newcastle council and the state government pays much, much more to put on that super car thing in Newcastle each year - we'll never know the real cost because council refuses to say - but 2.5 mill seems like a bargain for an event that could draw up to 160k visitors for 8 games (20k per game).
Because we are close to Sydney, half the visitors could be day visitors, with maybe free train fare linked to game tickets like you get for the easter show or other big events.
We will have all the new training grounds and facilities available by that time, so everything would be in place to host all teams, especially since all the Sydney based teams wouldn't need accommodation because it's a day trip for them.
It looks doable.

Can’t believe how poorly Eels are at moment, serious problems when thrashed rather than beaten.
As for Penrith, didn’t watch the game but Cleary was on hand for most of tries scored indicating effort when I saw highlights, very poor team effort.